The present invention is related to electronic components and methods of making electronic components. More specifically, the present invention is related to electronic components, and methods of making electronic components, with improved terminations for attachment of an external lead or lead frame to the electronic component such that the component can subsequently be connected to an electronic circuit or as a thermal heat sink bond to an aluminum or aluminum alloy heat sink.
In general, the method of formation of a conductive termination, and materials used, is critical for reliable performance. The performance in use, when subsequently assembled in an electronic circuit, is directly related to the conductive termination. Historically, lead (Pb) based solders have been used to attach components to electronic circuit boards or to attach external leads to the electronic component. More recently, the use of hazardous substances in electrical and electronic equipment, as typified by the European RoHS legislation, has restricted the use of lead (Pb) in solder and has led the industry to seek various alternatives.
U.S. Pat. No. 6,704,189, for example, describes the use of Sn based solder with 10-30% Sb to form a contact between external leads and plated Multi-Layer Ceramic Capacitor (MLCC) components. However, the solders described have a liquidous below 270° C. By way of comparison, high-Pb solders such as Sn10/Pb88/Ag2 have a liquidous of about 290° C. It is generally recognized in the industry that a melting point 30° C. above any subsequent processing temperature is desirable to insure reliability of the external lead attachment. The ability to achieve high melting points has become critical since solders based on Sn, Ag and Cu, which are referred to in the art as SAC solders, are now becoming the common choice for attachment in Pb-free circuits. SAC solders have to be reflowed at higher temperatures, typically about 260° C., than the older Pb-based alternatives such as Sn63/Pb37 which had a melting point of 183° C. The contact material to the external lead, or for forming the terminal, must be capable of sustaining temperatures well above this in order not to melt, or partially melt, which causes significant reliability issues. A temperature of at least 30° C. above the melting point of the SAC solder is desired.
U.S. Pat. No. 5,038,996 describes coating two mating surfaces, one with Sn and the other with Pb, and forming a joint by raising the process temperature to a temperature slightly below Sn (183° C.). Transient Liquid Phase Sintering conductive formulations disclosed in U.S. Pat. No. 5,853,622 combine TLPS materials with cross linking polymers to create a thermally and electrical bond having intermetallic interfaces between the metal surfaces created by the TLPS process. The spraying of one mating surface with a low temperature melting material and the mating surface with a higher melting temperature material and being compatible with the TLPS process and forming a joint when heating to the melting point of the lower temperature material is discussed in U.S. Pat. No. 5,964,395. These patents describe the materials and processes of TLPS with respect to forming a conductive bond.
U.S. Pat. No. 5,221,038 claims the use of SnBi or SnIn for soldering discrete components such as resistors and the like to printed circuit boards using the TLPS process. The use of Ag/SnBi coated to two mating surfaces to mount electronic modules to substrates was disclosed in U.S. Pat. No. 6,241,145. US Patent Application No. 2002/0092895 A1 discusses the deposition of materials on two mating surfaces, a substrate and the surface of the bumps on a flip chip, elevated to a temperature to cause diffusion between the materials to create a TLPS compatible alloy. US Patent Application No. 2006/0151871 describes the use of TLPS in forming packages containing SiC or other semiconductor devices bonded to other components or conductive surfaces. US Patent Application No. 2007/0152026 A1 claims the placement of TLPS compatible materials on mating surfaces then reflowing the lower melting point material and then isothermally aging to complete the diffusion process where the two devices to be joined are a MEMS device to a microelectronic circuit. U.S. Pat. No. 7,023,089 B1 claims the use of TLPS to bond heat spreaders made from copper, black diamond, or black diamond copper composite to silicon die. These patents and applications describe the processing of TLPS to bond components to circuit boards but do not contain any teaching regarding their use to form terminations on electronic components or in the attachment of components to lead frames.
In a more recent development US Patent Application No. 2009/0296311 A1 describes a high temperature diffusion bonding process that welds the lead to the inner electrodes of a multi-layer ceramic component. TLPS materials are plated on the faces of mating surfaces to be joined together by introducing heat to initiate the diffusion process. In this case an intimate mutual line of contact is required between the component and lead frame to facilitate the diffusion. This limits the application to the joining of surfaces that can form an intimate line of contact and also cannot accommodate components of differing length. Furthermore, high temperatures in the range 700 to 900° C. are described to achieve a welded bond that requires pre-heating in order not to damage the multi-layer ceramic component.
Other Pb free attachment technologies are described in the art yet none are adequate.
Solder is an alloy consisting of two or more metals that have only one melting point, which is always lower than that of the metal having the highest melting point and generally, a melting point of less than about 310° C., depending on the alloy. Solder can be reworked, meaning it can be reflowed multiple times, thus providing a means to remove and replace defective components. Solders also make metallurgical bonds by forming intermetallic interfaces between the surfaces they are joining. As solders wet to their adjoining surfaces, they actually flow outward and spread across the surface areas to be joined.
Due to environmental issues associated with lead; tin, silver, and copper solders known as lead free SAC solders were developed for electronics but these are typically reflowed at peak temperatures around 260° C. and the electronic components therefore have to be operated below this temperature. Due to materials compatibility and higher processing temperatures involved with the semi-conductor technologies, gold/germanium and gold/tin alloys were developed to attach dies to substrates. Since the die and their mating surfaces have a low difference in thermal coefficient of expansion TCE, these alloys provide high temperature capabilities and high strengths having tensile strengths in the range of 20,000 psi+ and shear strengths in the range of 25,000 psi+. However, these materials also require higher processing temperatures due to their higher melting points, generally above 350° C. Their high cost and limited application has prevented their wider use in electronics. Tin and indium have been added to combinations of Zn, Al, Ge and Mg to form higher temperature lead free solders. However, zinc and aluminum powder tend to form oxide films on the surface resulting in poor wettability in the subsequent solders making them impractical to use. Solders with tin, zinc, cadmium, and aluminum are available but typically used in their eutectic alloy form because their alloys, other than eutectics, have wide plastic ranges of 50-175° C. thereby limiting their use to very specific applications outside of the electronics. Cadmium, zinc, and silver alloy solders are good for soldering aluminum or aluminum alloy. Once the liquidus temperatures move above about 450° C. the solders are referred to as brazing solders and are typically used in structural applications rather than electrical applications. Methods of forming Pb-free, high temperature bonds to capacitors that retain their integrity above 260° C. and which are economic to manufacture have therefore yet to be realized.
A particular problem in the art is associated with soldering of aluminum conductors such as aluminum anodes, aluminum cathodes or aluminum heat sinks to other aluminum conductors or to leads. Aluminum is known to form a surface oxide almost immediately in normal air and it is difficult to form a metallurgical bond through the oxide. Those of skill in the art have been forced to utilize fluxes in solders wherein the fluxes are defined herein as materials which remove the oxides and allow for adequate bond formation. Unfortunately, the components of the flux remain as impurities in the bond which is detrimental to the bond resulting in a lower bond density. There has been no adequate solution to this problem thereby limiting the applications which can rely on the electrical and thermal advantages of aluminum.
In spite of the ongoing, and intensive effort, the art still lacks an adequate solution for attaching external leads or lead frames to a multi-layered ceramic capacitor. There is an ongoing need for lead connections with improved reliability for high temperature applications, especially lead (Pb) free and especially with aluminum.